WebPEncode: Support encoding same pic twice (even if modified)

This wasn't working for this specific scenario:
- Encode an RGBA 'pic' (with trivial alpha) using lossy encoding.
(so that pic->a == NULL after import happens).
- Modify the 'pic->argb' so that it has non-trivial alpha.
- Encode the same 'pic' again.
This used to fail to encode alpha data as pic->a == NULL.

Change-Id: Ieaaa7bd09825c42f54fbd99e6781d98f0b19cc0c

src/enc/webpenc.c

@@ -326,7 +326,7 @@ int WebPEncode(const WebPConfig* config, WebPPicture* pic) {
 
   if (!config->lossless) {
     VP8Encoder* enc = NULL;
-    if (pic->y == NULL || pic->u == NULL || pic->v == NULL) {
+    if (pic->use_argb || pic->y == NULL || pic->u == NULL || pic->v == NULL) {
       // Make sure we have YUVA samples.
       if (config->preprocessing & 4) {
         if (!WebPPictureSmartARGBToYUVA(pic)) {